There were cries of "Help" and "Anyone please help" before a woman emerged from the collapsed tunnel in Japan, barefoot and trembling, knowing her boyfriend and all her friends were trapped inside.

It is one of the many scenes of horror described by survivors of the tunnel collapse on the Chuo Expressway, which passes through hills near Mount Fuji, on Sunday.

At least nine people have been killed.

Witnesses told of terrifying scenes as at least one vehicle burst into flames, sending out clouds of blinding, acrid smoke.

They also spoke of scenes of confusion in the nearly five-kilometre tunnel, as cars drove the wrong way to try to escape the cave-in and fire.

One man told the Jiji Press news agency he had watched in horror as concrete crashed down onto a vehicle in front of him, leaving little more than a mound of dust and debris.

Voices cried out "Help" and "Anyone please help" from the pile before a young woman emerged, barefoot and with her clothes torn, he was reported as saying.

She could not stop trembling, he told the agency, as he asked her how many had been inside the vehicle.

"She said: 'All of my friends and my boyfriend...Please help them."' said the man.

He said the flames were too strong.

Tomohiro Suzuki, 37, said he had jumped out of his car and rushed towards a vehicle that had been crushed by the fallen concrete, in a bid to rescue people inside.

"A part of the ceiling, just as wide as the road, had collapsed straight down and broken in the middle into a V-shape," he told Jiji, adding there was a fire in its bonnet.

"I called the police as I thought it was impossible for me to rescue anyone inside."

Suzuki said people in the tunnel were in panic, with cars starting to drive the wrong way to try to get out.

He gathered up his 38-year-old wife, Nishiki, and their two children, six and nine, and hurried them to safety as several dozen people rushed to get out.

"I was anxious as I could not see the exit," he said.

"I kept wondering when the fire would spread and catch us. I tried to let my children get away first of all."

Suzuki, from central Nagano prefecture, said it had taken him and his family about an hour to get out of the tunnel.

By the time they got out, the fire had established itself and they could see smoke coming from the tunnel mouth.

London's Daily Telegraph quoted a woman, who told public broadcaster NHK: "I could hear voices of people calling for help, but the fire was just too strong."

Another man told NHK said concrete started falling on his roof as he was driving.

"I saw a car that had been crushed and had caught fire," he said.

"I was frightened. I got out of my car and walked for about one hour to get out of the tunnel."

Another driver who survived was Yoshio Goto, a reporter for NHK, who said he accelerated to escape the cave-in.

"I was a bit too late and pieces of the roof fell on my car," he said. "I kept pressing the pedal and managed to get out. Then, when I looked around, I saw that half of the top of my car had been crushed."

Dozens of people abandoned their vehicles on the Tokyo-bound section of carriageway, and ran for one of the emergency exits or for the mouth, where they huddled in bitter winter weather.

Emergency crews equipped with breathing apparatus battled around a third of the way into the tunnel, where they found 110 metres of concrete panels had come crashing down, crushing at least two vehicles.

Hours after the collapse, engineers warned the structure could be unstable, forcing rescuers to halt their work as a team of experts assessed the danger.

It was during this inspection that accompanying police officers confirmed the first deaths.

"What we found resembled bodies inside a vehicle, they were blackened. We have visually confirmed them but have yet to take them out for closer examination," an official told AFP.

The Fire and Disaster Management Agency later confirmed there were five bodies, adding another vehicle had also been burned.

Chikaosa Tanimoto, professor emeritus of tunnel engineering at Osaka University told NHK the concrete panels are suspended from pillars.

"It is conceivable that the parts connecting the ceiling panels and pillars, or pillars themselves, have deteriorated, affected by vibrations from earthquakes and passing vehicles," he said.

An official from highways operator NEXCO said material degradation was a possibility, adding the risk of further collapse remained. His colleague said the ceiling had undergone its regular five-yearly inspection in September this year.

The tunnel is one of the longest in Japan. It sits on a major road connecting Tokyo with the centre and west of the country.

Japan has an extensive web of highways with thousands of tunnels, usually several hundred metres long. Millions of cars use the network every day.